The present invention relates to drill bits used in oil and gas drilling, and especially to the use of inserts and hardfacings thereon.
Oil wells and gas wells are drilled by a process of rotary drilling, using a drill rig such as is shown in FIG. 4. In conventional vertical drilling, a drill bit 410 is mounted on the end of a drill string 412 (drill pipe plus drill collars), which may be miles long, while surface equipment 414 turns the drill string, including the bit at the bottom of the hole.
Two main types of drill bits are in use, one being the roller cone bit, an example of which is seen in FIG. 6. In this bit a set of cones 616 (two are visible) are arranged on rugged bearings such that when the drill string to which they are connected is rotated, each cone will separately rotate about its separate axis along the bottom of the borehole. The cones will generally have either milled teeth cut out of the same steel of which the cones are made or inserts 12 of a harder material than the steel cone. The teeth generally work in a gouging, scraping motion to remove softer formations, while inserts are generally preferred for harder formations, where their hardness, combined with the weight on the bit, acts to fracture the rock, which is then swept by the circulating mud.
The second type of drill bit is a drag bit, having no moving parts, seen in FIG. 5. These bits are increasingly popular, especially in softer formations. Like rotary cone bits, they can also carry either milled teeth or harder inserts.
During drilling operations, drilling fluid, commonly referred to as xe2x80x9cmudxe2x80x9d, is pumped down through the drill string and out nozzles 628 in the drill bit. The flow of the mud is one of the most important factors in the operation of the drill bit, serving to remove the cuttings which are sheared from rock formations by the drill bit, to cool the drill bit and teeth, and to wash away accumulations of soft material which can clog the bit.
When the bit wears out or breaks during drilling, it must be brought up out of the hole. This requires a process called xe2x80x9ctrippingxe2x80x9d: a heavy hoist pulls the entire drill string out of the hole, in stages of (for example) about ninety feet at a time. After each stage of lifting, one xe2x80x9cstandxe2x80x9d of pipe is unscrewed and laid aside for reassembly (while the weight of the drill string is temporarily supported by another mechanism). Since the total weight of the drill string may be hundreds of tons, and the length of the drill string may be tens of thousands of feet, this is not a trivial job. One trip can require tens of hours and is a significant expense in the drilling budget. To resume drilling the entire process must be reversed. Thus the bit""s durability is very important, to minimize round trips for bit replacement during drilling.
The inserts or compacts used in drill bits are made of a super-hard material and a softer binder material which are formed into the desired shape at very high temperatures and pressures. The normal composition is tungsten carbide and from 5 to 17% cobalt in a sintered condition. The inserts are press-fitted into holes drilled into the drill bit. In recent years inserts have been added to areas of steel tooth roller cone bits. A common problem associated with the placement of these inserts and those on other bits is that the steel holding the insert in place erodes and leaves the insert in peril of damage or simply falling out from its pocket. FIG. 7 gives an example of how erosion weakens the area around an insert. In this drawing, body areas 710 adjacent the insert 12 have been washed away by the scouring action of abrasives carried in the drilling mud. If the insert does come out, it does more than simply leave the bit with one less cutting element. The inserts tend to be very dense and is not easily carried away by the drilling mud. Rather, it tends to stay near the bottom of the hole, where its hard nature causes it to knock out following teeth and to quickly destroy the bit. Often, not only must the bit be replaced, but fishing tools can be necessary to remove broken teeth from the hole prior to the resumption of drilling.
It has become common to coat areas of a bit which are subject to erosion with a layer of hardfacing. An exemplary hardfacing comprises tungsten carbide in a matrix of steel, which is applied to the finished bit by welding. While hardfacings are being used in many high-wear areas of the bit, it has been difficult to apply hardfacings near inserts. This is because the inserts, which are press-fitted into their holes, are often loosened by the heating used in the application of the hardfacing, a counter-productive activity. To prevent this, a margin has generally been left around the inserts when hardfacing is applied.
Thermal spray coatings, such as that shown in U.S. Pat. No. 5,535,838, have been applied over the cutting structure but because of the relative thin layer (0.005xe2x80x3 to 0.020xe2x80x3) there is minimum protection of the insert and no contribution to the metallurgical bonding of the insert. Thermal spray coatings are not appropriate for steel tooth roller cone rock bits or areas of high wear such as the gage area of a fixed cutter bit or the surf or backface area of a roller cone.
Improved Wear Protection on Rock Bits
The present inventor has realized that wear protection on rock bits can be enhanced by combining features of superhard inserts and deposited hardfacing layers. In order to combine the two, the inserts are welded into place. They may be welded using a hardfacing or weld material or a combination of both. The compacts used can be ordinary compacts or they can be specifically modified to improve their weldability. After the inserts are welded into place, additional hardfacing material is applied to the bit, including the areas immediately adjacent to and even over the inserts. This method produces parts with inserts which are not destroyed during hardfacing and heat treat, as opposed to present methods in which either hardfacings OR pressed inserts alone are used in these high wear areas.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
ability to intimately combine inserts and hardfacings;
metal supporting the insert is protected from erosion;
inserts are less likely to come out of socket;
savings in time/money from retention of inserts.